SnO2 having different surface areas were synthesized using a surfactant-templating synthetic method. The effects of surface areas of the obtained SnO2 on their gas sensing and catalytic properties were investigated. Interestingly, a linear relationship was found between the surface areas of SnO2 and their gas sensitivities to H2 and CO. Furthermore, a linear relationship was observed between the surface areas and the catalytic activities of SnO2 in the oxidation of H2 and CO. Based on the fact that the mechanisms for catalytic reactions and gas-sensing processes both involve surface adsorption and reaction between surface-adsorbed oxygen species and surrounding gases (H2 or CO), the relationship between these two mechanisms occurring on SnO2 surfaces is discussed.